1) Field of the Invention
The present invention relates to an interface apparatus for SDH/SONET interconnection for mutually connecting apparatuses of two different systems SDH and SONET.
2) Description of the Related Art
Presently, user network interfaces for realizing a B-ISDN (Broadband aspects of Integrated Services Digital Network) are standardized in accordance with a unified standard called SDH (Synchronous Digital Hierarchy) so as to unify digital hierarchies existing in various countries throughout the world.
Under SDH, all information signals from different countries throughout the world, which have different transmission speeds (frame formats), are converted into information signals having a transmission speed of 150 Mbps (more specifically, 155.52 Mbps) series and are multiplexed, thereby making it possible to transmit freely among various countries information signals having different transmission speeds.
Before introduction of SDH as a worldwide standard, Japan employed a hierarchy based on a transmission speed of 1.5 Mbps (24.times.64 kbps) series in which 24 signals of 64 kbps each resulting from digital conversion of a voice signal of 4 kHz were multiplexed, while European countries employed a hierarchy based on a transmission speed of 2 Mbps (30.times.64 kbps) series in which 30 signals of 64 kbps were multiplexed.
In the U.S., SONET (Synchronous Optical Network) based on a transmission speed of 50 Mbps (13.times.60.times.64 kbps) series was independently used as a standard hierarchy.
Since the base transmission speeds differ among the countries as described above, information could not be freely transmitted among the countries.
To solve this problem, SDH employs a method for unifying the various digital hierarchies. In this method, a virtual "box" called a virtual container (VC) is defined, and several information signals having low transmission speeds (information of a lower order) are put into the "box", and a plurality of such "boxes" are collected and put into a larger "box". Using this method, information signals having different transmission speeds can ultimately be inserted into a single large "box" for transmission.
For example, when information signals are of 1.5 Mbps series, four information signals are inserted into a "box" having a capacity of 6 Mbps, and seven of such 6 Mbps "boxes" are inserted into a "box" having a capacity of 50 Mbps. Further, three of such 50 Mbps "boxes" are inserted into a single "box" having a capacity of 150 Mbps and then transmitted.
When information signals are of 2 Mbps series, three information signals are inserted into a "box" having a capacity of 6 Mbps, and several such 6 Mbps "boxes" are inserted into a larger "box". This procedure is repeated, so that the information signals are ultimately inserted into a single "box" having a capacity of 150 Mpps and are transmitted. Similarly, three information signals of 50 Mbps series standardized under SONET of the U.S. are inserted into a "box" having a capacity of 150 Mbps and then transmitted.
Information having a converted transmission speed (frame format) of 150 Mbps series is called STM-1 (Synchronous Transport Module Level 1) under SDH, and is called STS-3c under SONET.
FIGS. 36(a) and 36(b) are diagrams showing examples of the frame formats of STM-1 and STS-3c signals, respectively. As shown in FIGS. 36(a) and 36(b), the frame format of the STM-1 and the frame format of the STS-3c are basically the same, and each has a format structure of 9 rows.times.270 bytes consisting of an overhead portion called SOH (Section Overhead) of 9 rows.times.9 bytes, and a data portion (SPE) of 9 rows.times.261 bytes.
Information having a transmission speed equal to or greater than 150 Mbps has a frame format in which a plurality of STM-1 signals or STS-3c signals (total number is an integer) are multiplexed, and is represented as STM-n or STS-m (n and m are degrees of multiplexing) as shown below:
______________________________________ SDH SONET ______________________________________ STM-1 = STS-3c STM-4 = STS-12 . . . . . . STM-64 = STS-192 . . . . . . STM-n = STS-m ______________________________________
SDH and SONET are basically the same in their frame lengths and frame formats, as shown in FIGS. 36(a) and 36(b). However, in a high speed mode whose operating speed is equal to or higher than that of STM-64/STS-192, they differ from each other in the definition of a frame synchronization pattern included in the first row of the SOH. Further, they differ from each other in the definition of a portion called SS bits in a pointer byte which is included in the fourth row of SOH and represents the leading position of the SPE (the SS bits are "00" in SONET but "10" in SDH), and also differ from each other in the definition of an undefined byte (the undefined bytes are all "0" in SONET but all "1" in SDH).
Therefore, when an apparatus of the SDH system and an apparatus of the SONET system are mutually connected for operation, an alarm is generated due to a partial disagreement between their formats, so that it becomes impossible to perform communication. Consequently, it becomes impossible to interconnect these apparatuses of different systems.
An example is assumed in which apparatuses of the SDH system (an interface apparatus for SDH) such as an SDH multiplexing apparatus 101A and an STM-1 format conversion apparatus 102A are interconnected with apparatuses of the SONET system (an interface apparatus for SONET) such as a SONET multiplexing apparatus 101B and an STS-3c format conversion apparatus 102B, while a signal having a transmission capacity of 150 Mbps is transmitted through the multiplexing apparatuses 101A and 101B.
As shown in FIG. 39, each of the SDH multiplexing apparatus 101A and the SONET multiplexing apparatus 101B comprises a transmission section 107 consisting of a multiplexing unit 103, an overhead inserting unit 104, a frame pattern inserting unit 105 and an electricity/light converting unit 106; and a reception section 113 consisting of a light/electricity converting unit 108, a frame synchronization processing unit 109, an overhead processing unit 110, a separating unit 111 and an alarm processing unit 112.
In the transmission section 107 of the SDH multiplexing apparatus 101A, a plurality of STM-1 signals to be transmitted are multiplexed by the multiplexing unit 103 together with other equivalent signals (STM-1 signals), so that they are converted into an STM-n signal (n is the degree of multiplexing). For this STM-n signal, overhead inserting processing and frame synchronization pattern inserting processing are performed in an SDH mode within the overhead inserting unit 104 and the frame pattern inserting unit 105, respectively. Consequently, the STM-n signal is converted into a light signal by the electricity/light converting unit 106 and is transmitted toward the counterpart apparatus.
Similarly, in the transmission section 107 of the SONET multiplexing apparatus 101B, a plurality of STS-3c signals to be transmitted are multiplexed by the multiplexing unit 103 together with other equivalent signals (STS-3c signals), so that they are converted into an STS-m signal (m is the degree of multiplexing). For this STS-m signal, overhead inserting processing and frame synchronization pattern inserting processing are performed in a SONET mode within the overhead inserting unit 104 and the frame pattern inserting unit 105, respectively. Consequently, the STS-m signal is converted into a light signal by the electricity/light converting unit 106 and is transmitted toward the counterpart apparatus.
In the reception section 113 of the SDH multiplexing apparatus 101A and the reception section 113 of the SONET multiplexing apparatus 101B, an STM-n signal or an STS-m signal transmitted from a counterpart multiplexing apparatus is converted into an electrical signal by the light/electricity converting unit 108. The frame synchronization of the converted signal is detected by the frame synchronization processing unit 109 so that synchronization is established. After the signal is subjected to processing for replacing the overhead or other processing performed by the overhead processing unit 110, the signal is separated by the separating unit 111 into a plurality of STM-1 signals or STS-3c signals which correspond to those existing before multiplexing.
As described above, the frame formats of the STM-1 and STS-3c differ from each other in the definition of the SS bits of the pointer byte in the overhead section. That is, the SS bits are "10" in SDH but "00" in SONET. Therefore, each of the reception sections 113 of SDH multiplexing apparatus 101A and the SONET multiplexing apparatus 101B receives an SS bit value different from the defined value therefor.
Therefore, in the reception section 113, when a received signal is processed by the overhead processing unit 110, it is judged that the value of the SS bits is an invalid pointer value. In this case, an LOP (Loss of Pointer) alarm is generated by the alarm processing unit 112, so that it becomes impossible to continue communication.
In a high speed mode in which signals each having a transmitting capacity equal to or greater than that of STM-64 (STS-192) are exchanged, SDH and SONET differ from each other in their frame synchronization patterns, as has been described with reference to FIG. 37. Therefore, there is a possibility that frame synchronization cannot be established within the frame synchronization processing unit 109, resulting in lack of synchronization.
Accordingly, as shown in FIG. 40(c), it is practically impossible to interconnect an apparatus of the SONET system with an apparatus of the SDH system so as to operate them, although it is possible to establish interconnection between apparatuses of the same system, i.e., the SONET system or the SDH system so as to operate them (or establish communication therebetween), as shown in FIG. 40(a) and FIG. 40(b).